1. Field of the Invention
This invention relates generally to a method for determining the membrane protonic resistance of a fuel cell stack using high frequency resistance and, more particularly, to a method for determining the membrane protonic resistance of a fuel cell stack using high frequency resistance by determining how much of the total high frequency resistance is caused by various non-membrane resistances and then removing these resistances.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, a gas diffusion layer (GDL) that protects the mechanical integrity of the membrane and helps in uniform reactant and humidity distribution. The part of the MEA that separates the anode and cathode flows is called the active area, and only in this area the water vapors can be freely exchanged between the anode and cathode. MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates (separators) positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include anode side and cathode side flow distributors (flow fields) for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Fuel cell membranes are known to have a water-uptake which is necessary to provide proton conductivity. The water-uptake behavior of fuel cell membranes, however, causes an increase of volume of the membranes if conditions become more humid or wet and a decrease of volume if conditions become dryer. Changes in the volume of the cell membranes may cause mechanical stress on the membrane itself and the adjacent fuel cell components. In addition, a membrane that is too wet may cause problems during low temperature environments where freezing of the water in the fuel cell stack could produce ice that blocks flow channels and affects the restart of the system. Membranes that are too dry may have too low of an electrical conductivity at the next system restart that affects restart performance and may reduce stack durability.
It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas of a fuel cell stack, and use the water to humidify the cathode input airflow. It is also known in the art to use relative humidity (RH) sensors to monitor the humidification of the cathode input airflow. However, RH sensors can be costly and unreliable. Therefore, there is a need in the art to provide a method for maintaining an appropriate level of cell membrane humidification without relying on RH sensors.
High frequency resistance (HFR) is a well-known property of fuel cells, and is closely related to the ohmic resistance, or membrane protonic resistance, of fuel cell membranes. Ohmic resistance is itself a function of the degree of fuel cell membrane humidification. Therefore, by measuring the HFR of the fuel cell membranes of a fuel cell stack within a specific band of excitation current frequencies, the degree of humidification of the fuel cell membrane may be determined. This HFR measurement allows for an independent measurement of the fuel cell membrane humidification, thereby eliminating the need for RH sensors. However, variations in HFR measurements of the cell membranes can occur from stack to stack for various reasons, including HFR measurement errors (bias), variations in stack materials, or variations in stack design or compression. Furthermore, variations in HFR measurements can also occur due to degradation of stack components over the life of the fuel cell stack, such as delamination, which causes an increase in contact resistance. These variations in HFR measurements may be cumulatively referred to as non-membrane contact resistances. Non-membrane contact resistances are considered “noise” and may lead to inaccuracies in determining cell membrane humidification. Non-membrane contact resistance, may be quite large, e.g., 20 miliohms-cm2.
Accordingly, there is a need in the art to calculate how much of the HFR measurement is caused by various non-membrane contact resistances to enable the control system of the fuel cell stack to remove the HFR that is caused by non-membrane contact resistances to determine how much of the total HFR measurement is caused by the protonic resistance of the fuel cell membrane, also known as the base fuel cell stack high frequency resistance.